Tube bundle heat exchangers are used in a number of applications, and have been extensively used in automotive applications. Such heat exchangers typically comprise a bundle of spaced, parallel tubes enclosed in a housing or shell. A first heat exchange fluid flows through the tubes, while a second heat exchange fluid flows through the housing and passes through the interstitial spaces between the outer surfaces of the tubes.
In a typical construction of a tube bundle heat exchanger, parallel tubes of circular cross-section are retained in place at their ends by perforated header plates, also known as tube sheets. In addition to retaining the tubes, the header plates also provide a seal to prevent flow communication between the tube interiors and the interior of the housing. The seal between the tubes and the header plate is usually provided by welded or brazed butt joints between the side surfaces of the tubes and the peripheral edges of the perforations in the tube sheet. Similarly, the header plate is sealed to the inner surface of the shell by a welded or brazed butt joint. Such joints provide a relatively small sealing surface and are prone to stress-induced failure. High stresses caused by thermal cycling effects are of particular concern in high temperature heat exchangers such as exhaust gas recirculation (EGR) coolers and fuel reformer heat exchange devices.
The incidence of stress-induced failure can be reduced by increasing the thickness of the header plate, thereby increasing the surface areas of the joints between the header plate and the tubes and between the header plate and the shell. However, increasing the thickness of the header plate by a significant amount adds to the material cost and significantly increases the cost of tooling and the complexity of forming the holes in the header plate.
Furthermore, one of the performance-driven goals of heat exchanger design is the reduction of tube diameters to increase fluid flow rates and heat transfer rates. However, conventional tube bundle heat exchangers cannot easily accommodate small diameter tubes due to the complexity of stamping small-diameter holes, and the compounding difficulty of forming the holes in thicker header plate constructions.
It is known to construct tube bundle heat exchangers without conventional header plates. For example, header plates can be eliminated by providing tubes with expanded ends shaped to directly engage and nest with one another while maintaining the central portions of the tubes in parallel, spaced relation to one another. Examples of this type of heat exchanger are cellular-type radiators of the type used in early automobiles and airplanes, and as described in Chapter 4 of “Automotive Cooling System Basics” by Randy Rundle, Krause Publications, 1999, pages 18 to 30. In cellular-type radiators, the ends are expanded to a shape which permits the tubes to be nested together. In use, air passes through the horizontal tubes and engine coolant flows down and around on the outsides of the tubes.
An exhaust gas cooler having a tube bundle comprising rectangular tubes with expanded ends is described in U.S. Pat. No. 6,321,835 to Damsohn et al. As shown in FIG. 1 of Damsohn et al., the expanded tube ends are connected to one another and to the heat exchanger shell. Although Damsohn et al. avoids use of perforated headers, it requires that the shell be formed with a complex shape for joining directly to the irregularly shaped tube bundle.
There is a need for improved constructions for tube bundle heat exchangers which preferably avoid the use of conventional, perforated header plates and/or conventional baffle plates.